Over the past decade, there has been a marked increase in both the number of new cases and in deaths related to lung cancer nationwide. Tobacco (cigarette smoking) has been implicated in the initiation and promotion of the majority of these new cases of lung cancer. Carcinogens and tumor promoters in cigarette smoke are thought to be responsible for many of the genetic and epigenetic alterations in bronchial and alveolar epithelial cells that result in the multistep progression into lung cancer.
Peripheral adenocarcinoma (PAC) and bronchioloalveolar carcinoma (BAC) are forms of lung cancer whose etiology and pathogenesis are controversial and whose link to either main stream tobacco smoking or second hand smoking unproven. While squamous cell carcinomas and small cell carcinomas have shown an overall decrease in incidence in the past decade, peripheral adenocarcinomas (PACs) and bronchioloalveolar lung cancers (BACs) have shown an exponential increase (Barsky et al., Cancer 1994; 73: 1163–1170; Barsky et al., Modern Path 1994; 7: 633–640). These increases have been observed equally in both smokers as well as non-smokers. These epidemiological observations provide evidence that either different etiological factors exist (other than main stream or second hand smoke) that cause PACs and BACs or that different etiological co-factors that are synergistic with main stream or second hand smoke play a role in the genesis of PAC/BAC. Some of the distinguishing pathological, biological epidemiological and perhaps etiological features which distinguish PAC/BAC from other types of lung cancer are its peripheral location, its association with desmoplasia (scarring) (Barsky et al., Am J Pathol 1986; 124:412–419), its significant occurrence in non-smokers, its comparatively high female/male ratio, and its tendency to appear in multiple and bilateral foci (especially BAC). Although some theories suggest that this multifocality is due to intrapulmonary metastatic spread or a combination of aerosolization and aspiration, we have demonstrated that at least a partial basis for this multifocality or multicentricity is multiclonality (Barsky et al., Cancer 1994; 73: 1163–1170; Barsky et al., Modern Path 1994; 7: 633–640).
The multifocal nature of BAC was first described in 1876 Malassez et al., Archives of Physiology and Normal Pathology 1876; 3:353–372). BAC was further defined as a well-differentiated adenocarcinoma occurring in the periphery of the lung which tended to spread along aerogenous and lymphatic routes within the confines of the lung (Liebow, Bronchiolar-alveolar carcinoma. Adv Intern Med 1960; 10:329–358). In the revised WHO lung tumor classification (ICD-0) (World Health Organization. Histological typing of lung tumors. International histological classification of tumors, 2nd ed. Geneva. WHO. 1981), bronchioloalveolar carcinoma was included as a subtype of adenocarcinoma. Although there has been considerable debate as to whether bronchioloalveolar carcinoma represents a distinct clinical entity separate from other adenocarcinomas (Schraufnagel et al., Am Rev Resp Dis 1982; 125:74–79), recent evidence suggests differences in incidence trends, survival, and sex and age distributions compared to other adenocarcinomas and other lung cancer cell types (Grover et al., Ann Surg 1989; 209:779–90). For these reasons BAC should be considered as a distinct clinicopathological entity. Pathologists categorize BAC into mucinous and non-mucinous on the basis of their microscopic appearance and further subcategorize non-mucinous BAC into Clara, Type II pneumocyte and mixed cell of origin on the basis of ultrastructural and immunocytochemical findings (Yoneda K Cancer 1990;164–169).
Peripheral adenocarcinoma has long been the predominant cell type among female lung cancers (Wynder et al., Eur J Clin Oncol 1987; 23:1491–1496) and there is abundant evidence that this cell type has been increasing in both men and women (Vincent et al., Cancer 1977; 39:1647–1655; Dodds et al., JNCI 1986; 76:21–29; Valaitis et al., Cancer 1981;47:1042–1046; Percy et al., Lung cancer: causes and prevention. Mizell M, Correa P, eds. Verlag Chemie International Inc. 1984). According to SEER data for 1973–1981 (Percy et al., Lung cancer: causes and prevention. Mizell M, Correa P, eds. Verlag Chemie International Inc. 1984), the incidence of adenocarcinoma has increased 3% per year in males compared to 1.50% per year in females. In 1981, 25% of white male lung cancers and 35% of white female lung cancers were adenocarcinoma. Increases in the proportion of adenocarcinomas have also been reported in Japan (Tsugane et al., Jpn J Can Res 1987; 78:162–169; Watanabe et al., Jpn J Cancer Res 1987; 78:460–6) and Israel (Rennert et al., Cancer Det Prev 1991; 15:99–101). Few studies report the proportion of bronchioloalveolar carcinomas separately from other adenocarcinomas. In SEER data (Percy et al., Lung cancer: causes and prevention. Mizell M, Correa P, eds. Verlag Chemie International Inc. 1984), 17% of adenocarcinomas were bronchioloalveolar carcinomas, 15% were other subtypes, and 68% were classified adenocarcinoma NOS. While bronchioloalveolar carcinoma (BAC) represented approximately 3.5% of all lung cancers in the SEER data Percy et al., JNCI 1983; 70:663–666), other studies have reported this proportion to vary from 1% to 8% (Liebow “. Bronchiolar-alveolar carcinoma. Adv Intern Med 1960; 10:329–358; Thomas et al., Br J Dis Chest 1985; 79:132–140; Bennett et al., Cancer 1969; 24:876–887). Recently, there have been several reports of an increase in the incidence of bronchioloalveolar carcinomas. Among patients included in seven Lung Cancer Study Group protocols between 1977 and 1988, 14.5% met the classification criteria for bronchioloalveolar carcinoma (Grover et al., Ann Surg 1989; 209:779–90). In a 1988 study (Gazdar et al.,. Sem Onc 1988; 15:215–225), the authors noted that bronchioloalveolar carcinoma was being diagnosed with increasing frequency in the Washington-Baltimore area. In a study of 505 autopsied cases of lung cancer diagnosed between 1973 and 1989 in New Jersey, other authors (Auerbach et al., Cancer 1991; 68:1973–1977) found that the incidence of bronchioloalveolar carcinoma increased from 9.3% to 20.3%, more than any other cell type. A similar increase was reported in Japan (Ikeda et al., Lung cancer 1991; 7:157164). For the time period 1982–1985, 71% of all adenocarcinomas were classif as a the bronchioloalveolar subtype. It is not clear how much, if any, of the recent increases observed for BAC can be explained by changes in diagnostic criteria.
While lung cancer typically shows a strong male predominance, the male/female ratio varies with histologic type. The male/female incidence ratios vary from a high for squamous cell carcinoma (M/F=2.4) to a low for adenocarcinoma (M/F=1.4) (Anton-Culver et al., Cancer Res 1988; 48:6580–6583). The male/female ratio for bronchioloalveolar carcinoma has been reported to range from that observed in all adenocarcinomas to less than unity. Two studies (Bennett et al., Cancer 1969; 24:876–887; Ikeda et al., Lung cancer 1991; 7:157164) reported a lesser male predominance for bronchioloalveolar carcinoma compared to all lung cancers but similar to that for all adenocarcinomas. Among patients included in the Lung Cancer Study Group protocols, a male/female ratio of 1.4 was observed for bronchioloalveolar carcinoma compared to 1.7 for other adenocarcinomas (Grover et al., Ann Surg 1989; 209:779–90). Similarly, the male predominance among SEER cases was less for bronchioloalveolar carcinoma (1.2) than for other adenocarcinomas (1.8) (Percy et al., Lung cancer: causes and prevention. Mizell M, Correa P, eds. Verlag Chemie International Inc. 1984). Another study (Schraufnagel et al., Am Rev Resp Dis 1982; 125:7479) however, reported an excess of bronchioloalveolar carcinoma in females compared to males. The male predominance for bronchioloalveolar carcinoma is clearly less than for other types of lung cancer and evidence suggests that it is close to, and possibly less than, unity. Studies of lung cancer cell types consistently report a younger age at diagnosis for adenocarcinoma (including bronchioloalveolar carcinoma) than for other cell types in both men and women (Anton-Culver et al., Cancer Res 1988; 48:6580–6583; McDuffie et al., J Clin Epidmiol 1991; 44:537–544; Greenberg et al., JNCI 1984; 72:599–603; Tsai et al., Cancer Det Prev 1988; 11:235–238). Few studies have compared the age distribution for cases of bronchioloalveolar carcinoma alone with other cell types. Two studies (Liebow, Bronchiolar-alveolar carcinoma. Adv Intern Med 1960; 10:329–358; Storey et al., J Thorac Surg 1953; 26:331–403) described bronchioloalveolar carcinoma as occurring with higher frequency in younger ages compared to other cell types. Another study (Schraufnagel et al., Am Rev Resp Dis 1982; 125:74–79) noted the same age distribution for bronchioloalveolar carcinoma and other adenocarcinomas but younger than for squamous cell carcinomas. Contrary to these reports, the Lung Cancer Study Group (Grover et al., Ann Surg 1989; 209:779–90) reported that bronchioloalveolar carcinoma occurs more frequently in older patients than does adenocarcinoma. It is not clear at this time whether or not the age distribution for bronchioloalveolar carcinoma differs from other adenocarcinomas. Survival for bronchioloalveolar carcinoma compared favorably with other adenocarcinomas and large cell carcinoma in the Lung Cancer Study Group experience (Grover et al., Ann Surg 1989; 209:779–90). Survival rates during the first two years after diagnosis were also better than for squamous cell carcinoma, although survival rates were equal after two years.
Adenocarcinoma of the lung has been shown to be associated with cigarette smoking but not as strongly associated with smoking as squamous or small cell carcinomas (McDuffie et al., Cancer 1987; 59:1825–30; Anton-Culver et al., Cancer Res 1988; 48:6580–6583; Osann et al., Cancer Res 1991; 51:4893–4897; Schoenberg et al., Am J Epidemiol 1989; 130:688–695). Risk ratios for adenocarcinoma associated with cigarette smoking in New Jersey men and women were 4.8 and 3.6 respectively (Schoenberg et al., Am J Epidemiol 1989; 130:688–695). Relative risks have not been calculated for bronchioloalveolar carcinoma. One study (Ikeda et al., Lung cancer 1991; 7:157164) noted a similar rate of tobacco use among cases of bronchioloalveolar carcinoma as for the overall group of adenocarcinomas. Others reported a higher proportion of non-smokers among cases of bronchioloalveolar carcinoma than among other adenocarcinomas (Schraufnagel et al., Am Rev Resp Dis 1982; 125:74–79; Auerbach et al., Cancer 1991; 68:1973–1977) and lower mean pack-years of exposure to cigarettes compared to other cell types (Schraufnagel et al., Am Rev Resp Dis 1982; 125:7479). The Lung Cancer Study Group (Grover et al., Ann Surg 1989; 209:779–90) reported that patients with bronchioloalveolar carcinoma were significantly less likely to have a history of smoking than were other cases of adenocarcinoma. While it is clear that bronchioloalveolar carcinoma is at most weakly associated with smoking, it is not clear whether the association with smoking differs from that observed for other adenocarcinomas. Because the association with smoking is weak at best, smoking is an unlikely explanation for the recent increases observed.
Exposure to sidestream or secondhand smoke in the home and workplace have been shown to double the risk of lung cancer in non-smokers (Janerich et al., New Engl J Med 1990; 323:632–6). Passive smoking significantly increased risk for adenocarcinoma in a study of Chinese women (Lam et al., Br J Cancer 1987; 55:673–8). However, no significant increase in risk for adenocarcinoma with passive smoking was noted in another study (Wu et al., JNCI 1985; 74:747–751). An increase in lung cancer risk associated with exposure to cooking oil vapors has also been reported (Gao et al., In J Cancer 1987; 40:604609). The importance of passive smoke for bronchioloalveolar carcinoma has not been well studied.
Incidence of bronchioloalveolar carcinoma is increased in patients with scleroderma (Montgomery R D, Stirling G A, Hamer N A J. Bronchiolar carcinoma in progressive systemic sclerosis. Lancet 1964; 1:586–7) and is associated with parenchymal scarring and interstitial inflammation of the lung (Liebow “Bronchiolar-alveolar carcinoma” Adv Intern Med 1960; 10:329–358; Marcq et al, Am Rev Respir Dis 1973; 107:621–629). Although 60% of bronchioloalveolar carcinomas in one study had radiographic evidence of a prior lesion in the same location as the tumor, no cases gave a medical history of illness including emphysema, tuberculosis, pneumonia, or pulmonary thromboembolism which could have caused scarring (Schraufnagel et al., Am Rev Resp Dis 1982; 125:74–79). Experimental evidence suggests that the scarring may be the result of the cancer rather than the cause (Barsky et al., Am J Pathol 1986; 124:412–419). The Lung Cancer Study Group reported that patients with bronchioloalveolar carcinoma were less likely than other adenocarcinoma patients to have a history of chronic lung disease (Grover et al., Ann Surg 1989; 209:779–90). However, this variable was highly correlated with smoking. Contamination of indoor air by radon from soil, water, or building materials has been shown to be a potentially important cause of lung cancer (Samet J M., JNCI 1989; 81:745–757). Exposure to radon in homes appears to carry only a small increase in risk for non-smokers (Svensson et al., Cancer Res 1989; 49:1861–1865). When risks were estimated by histologic type, this study (Svensson et al., Cancer Res 1989; 49:1861–1865) also noted that adenocarcinoma had the lowest increase in risk associated with radon exposure in homes of any cell type. Small cell carcinoma has most frequently been reported as the predominant type of lung cancer in uranium miners exposed to radon (Samet J M., JNCI 1989; 81:745–757). Although there have been no reports of an increase in bronchioloalveolar lung cancer in uranium miners, contamination of indoor air could contribute to an excess of this cell type in women who are more likely to spend time at home. Occupational risk factors for bronchioloalveolar carcinoma have not been identified. One study (Schraufnagel et al., Am Rev Resp Dis 1982; 125:74–79) found no clear predominance of any occupational group in his series of bronchioloalveolar lung cancer cases. Squamous and small cell carcinomas have most frequently been associated with occupational exposures. However, an increase in the proportion of adenocarcinomas has been noted with exposure to asbestos, beryllium, and polyvinyl chloride (Ives et al., Am Rev Respir Dis 1983; 128:195–209). Because of the equal frequencies of this cancer in men and women, and because occupational exposures are more common in men than in women, it is unlikely that occupation is responsible for the recent increase in this disease.
Because cats and dogs get tumors that clinicopathologically resemble their human counterparts yet presumably are not exposed to the same types of exogenous carcinogenic factors, a study of their molecular alterations might be revealing. Bronchioloalveolar lung carcinoma (BAC) is an example of such a cancer which involves the lungs diffusely in all three species.
Unlike other forms of lung cancer, BAC naturally occurs in two non-human species: sheep and cats. Neither of these animals are exposed to main stream or second hand smoke. Human BAC closely resembles histologically an infectious endemic disease of sheep called jaagsiekte (Bonne et al., Am J Cancer 1939; 35:491–501; De la Heras et al., Eur Respir J. 2000 Aug.;16(2):330–2. The sheep form occurs as ovine pulmonary adenomatosis or jaagsiekte, a disease caused by an exogenous retrovirus (JSRV). In sheep, the disease manifests as a diffuse pulmonic adenomatosis commonly called SPA for short. The SPA complex is of particular interest, since: at least some, and probably all, forms are infectious; members of the SPA complex range in pathology from inflammatory or infiltrative to carcinomatous; and various forms of SPA, while almost certainly of viral etiology, can occur in both epizootic and enzootic form, thus resembling both the picture of classic viral transmission (epizootic) and that of natural tumor epidemiology (occurring at low rates, or enzootic) in which viruses are not ordinarily thought to be implicated. Retrovirus-induced pulmonary carcinoma in sheep was achieved experimentally about a decade ago (DeMartini et al., JNCI 1987; 79:167–177). The cell of origin that gives rise to sheep jaagsiekte is thought to be the Type II pneumocyte, a cell thought also to give origin to one subtype of human non-mucinous BAC. The exogenous virus thought responsible for jaagsiekte has been cloned and reliably distinguished from endogenous retroviral sequences (Palmarini et al., J of General Virology 1995: 76: 2731–2737; Palmarini et al., J of Virology 1996; 70: 1618–1623; Palmarini et al., J of Gen Virology 1996a; 77: 2991–2998; Bai et al., J of Virology 1996; 70: 3159–3168). The feline form naturally occurs in old (>10 years) pure bred Persian and Himalayan cats. The cat form of BAC has not been at all studied and is not endemic nor contagious. It occurs sporadically and spontaneously in older pure bred cats, especially Persian and Himalayan. With respect to its sporadic and spontaneous nature, it has more similarities to human BAC than does sheep BAC (jaagsiekte). Feline BAC is also thought to be of Type II pneumocyte origin.
As is known in the art, animals and humans are affected by a variety of related and/or common pathogens. For example cats are frequently infected with or are carriers of toxoplasmosis, an infectious organism which can seriously harm the developing human fetus in pregnant women. It has been difficult to grow toxoplasmosis in culture and develop a vaccine against this disease. But since cats are the natural host for this organism, it has been suggested that feline cells may provide a nurturing environment for growing organisms such as toxoplasmosis in vitro. Consequently, there is a need in the art for reagents such as mammalian cell lines that can be used in the examination of mammalian pathogens. In addition, there is a need in the art for reagents such as mammalian cell lines that can be used in the examination of mammalian cancers such as BAC. The invention disclosed herein meets these needs.